Abstract The purpose of the proposed Career Development Award is to promote my development as an independent clinical researcher with a focus on improving the health of individuals with type 2 diabetes through behavior change interventions. Currently, 10% of health care dollars are spent on type 2 diabetes, making it the most costly chronic disease in the US. The ability to deliver interventions using cutting-edge technologies holds tremendous promise for making evidence-based therapies more widely available. However, careful screening of intervention components is needed to ensure that potent interventions are being delivered. The goal of the proposed research is to optimize an online diet intervention for improving the blood glucose control of overweight individuals with type 2 diabetes. To achieve my career goals I need additional training in (1) diabetes and nutrition, (2) advanced study design and biostatistical methodology, (3) theories of health behavior change, and (4) use of technology to support interventions. The strong mentoring team will help me develop into an independent researcher tackling the critical issue of creating effective behavior change and lifestyle interventions. The proposed research will apply an innovative methodology, the multiphase optimization strategy (MOST), to efficiently examine the effects of the experimental treatment components on 12-month glycemic control in 144 adults with type 2 diabetes. This design provides the information of three trials with the time and expense of one. The core of the program, administered to all participants, includes web-delivered lessons derived from empirically supported techniques for improving behavioral adherence, including goal setting, problem solving, relapse prevention, menus, reminders, and coaching. I will test whether adding the following experimental components will enhance the intervention's ability to improve health outcomes: (1) active vs. periodic self-monitoring of dietary intake using an ?app?; (2) training vs. no training in psychological tools; and (3) a low carbohydrate vs. a standard-of-care diabetes diet. The goal of Aim 1 is to optimize the intervention. It is hypothesized that one level of each component?active self-monitoring, training in psychological tools, and low carbohydrate diet?will reduce HbA1c more than the other level of each component?periodic self-monitoring, no training in psychological tools, and a standard-of- care diabetes diet. Aim 2 will examine feasibility and acceptability of the intervention, such as assessing participant engagement and dietary adherence. Exploratory Aim 3 will examine secondary outcomes including changes in weight, mood, quality of life, hunger, food cravings, and the need for medication. The intervention will be designed to be easily disseminated and effective, positioning it to make progress in the treatment of type 2 diabetes. At the conclusion, the optimized intervention will be ready to be evaluated in a large-scale randomized controlled trial.